1. Field of the Invention
The present invention relates in general to a detecting element for a flow meter, and more particularly to a detecting element for a thermal flow meter, which is suitably used to determine an amount of intake air introduced into an internal combustion engine, for example.
2. Discussion of the Prior Art
In a known type of detecting element for a thermal flow meter, electrical conductors or leads are fixedly secured by means of an adhesive to a support on which an electrically resistive body is formed, such that the conductors and the electrically resistive body are electrically connected to each other through an adhesive layer formed of the adhesive. An example of this type of detecting element is disclosed in laid-open publication No. 56-96326 of unexamined Japanese Utility Model Application.
More specifically described referring to FIG. 1, the detecting element of the above type employs a ceramic tube 2 as the support, which is formed of alumina or other ceramic materials, and a platinum layer 4 as the electrically resistive body, which is obtained by fired forming a platinum thin film on the outer circumferential surface of the ceramic tube 2, and then suitably patterning the applied platinum film so that the obtained platinum layer 4 has a predetermined resistance value. A pair of electrical conductors or lead wires 6, 6 made of platinum are secured to opposite open end portions of the ceramic tube 2, with a platinum paste (8) as the adhesive applied to each of the opposite end portions of the tube 2. The masses 8 of the platinum paste are held in contact with respective end portions of the lead wires 6, 6, and the corresponding ends of the platinum layer 4, for electrical connection between the lead wires 6 and the platinum layer 4. The thus constructed detecting element is substantially entirely covered with a protective coating 10 made of glass, for example.
In practical use of the detecting element of the type as described above, the lead wires 6 are fixed to metallic terminals by welding, for example, so that the detecting element is placed in an intended position in a fluid passage. FIG. 2 illustrates one application of a thermal flow meter using two detecting elements 16 constructed as described above. The detecting elements 16 are disposed within a gas passage 14 which is defined by a pipe 12 such as an iron pipe, so as to detect an amount of a gaseous fluid which flows through the gas passage 14. In this case, each detecting element 16 is held in position within the pipe 12, such that the opposite end portions of the element 16, i.e., free end portions of the lead wires 6, 6 are fixed by welding, for example, to respective metallic terminals 20, 20, such as stainless rods. These metallic terminals 20 are inserted into the pipe 12, through respective electrically insulating members 18 which are fitted in holes formed through the cylindrical wall of the pipe 12. The metallic terminals 20 are connected to an external control circuit for determining the amount of the gaseous fluid through the passage 14.
In the known detecting element 16, however, excellent electrical conduction between the platinum layer 4 and the lead wires 6 through the masses 8 of the platinum adhesive is incompatible with excellent fixation of the lead wires 6 to the ceramic tube 2 by using the same adhesive masses 8. If preference is given to the electrical conduction as described above, the detecting element 16 suffers from a reduced degree of fixation of the lead wires 6 to the ceramic tube 2, i.e., insufficient bonding strength between the lead wires 6 and the ceramic tube 2. Consequently, the lead wires 6 tend to be removed off the platinum adhesive masses 8 when the wires 6 are welded to the metallic terminals 20, or the operating characteristics of the detecting element 16 are likely to be varied during its use. If preference is given to the fixation of the lead wires 6 to the ceramic tube 2, by increasing the amount of a glass component of the platinum paste (8), the detecting element 16 suffers from excessively low electrical conductivity between the platinum layer 4 and the lead wires 6.